## TL;DR

GitHub Actions is a CI/CD platform integrated into GitHub repositories. Workflows (YAML files in `.github/workflows/`) trigger on events (push, PR, schedule) and execute jobs on runners (Ubuntu, Windows, macOS). Free for public repositories.

## Core Explanation

Workflow syntax: `on:` defines triggers, `jobs:` defines steps. Runners: GitHub-hosted (2-core, 7GB RAM, free for public) or self-hosted. Marketplace: community-built actions (checkout, setup-node, deploy). Matrix builds: test across multiple versions/OS combinations. Secrets: encrypted environment variables (`${{ secrets.TOKEN }}`).

## Further Reading

- [GitHub Actions Documentation](https://docs.github.com/en/actions)